The Swiss Federal Laboratories for Materials Science and Technology (Empa) has announced a significant breakthrough that could pave the way for the industrialization of flexible, light-weight and low-cost cadmium telluride (CdTe) solar cells on metal foils.

Flexible thin film solar cells that can be produced by roll-to-roll manufacturing are a highly promising route to cheap solar electricity.

Empa researchers succeeded in increasing cell efficiency from below eight to 11.5% by doping the cells with copper, as they report in the current issue of Nature Communications.

"Flexible thin film solar cells have a huge potential in this regard because they require only a minimum amount of materials and can be manufactured in large quantities by roll-to-roll processing," Empa said in a statement.

Cadmium telluride (CdTe) cells are currently the cheapest in terms of production costs and have a market share second only to silicon-based solar cells. Grown mainly on rigid glass plates, these so-called superstrate cells have one drawback: they require a transparent supporting material that lets sunlight pass through to reach the light-harvesting CdTe layer, thus limiting the choice of carriers to transparent materials.

The inversion of the solar cells multi-layer structure  the so-called substrate configuration  would allow further cost-cuttings by using flexible foils that can be made of metal as a supporting material. With such a design, sunlight can enters the cell from the other side, without having to pass through the supporting substrate.

Up till now, however, CdTe cells in a substrate configuration on metal foil have thus far exhibited extremely low efficiencies well below 8%  a modest comparison to the recently reported record efficiency of 19.6% for a lab-scale superstrate CdTe cell on glass. Commercially available CdTe superstrate modules reach efficiencies of between 11 and 12%.

Copper doping for solar cells

Empa researchers found a way to increase the low energy conversion efficiency of substrate CdTe cells by successfully doping the semiconductor layer with minute amounts of copper (Cu)  an achievement that has until now eluded scientists in the past.

"People have tried to dope CdTe cells in substrate configuration before but failed time and again," said Ayodhya Nath Tiwari, head of Empa’s laboratory for Thin Films and Photovoltaics.

Tiwari’s team nevertheless tried using high-vacuum Cu evaporation onto the CdTe layer with a subsequent heat treatment to allow the Cu atoms to penetrate into the CdTe. The amount of Cu had to be painstakingly controlled: too little would not improve the efficiency but neither would using too much.

The electronic properties improved significantly, however, when Lukas Kranz, a PhD student in Tiwari’s lab, along with Christina Gretener and Julian Perrenoud, fine-tuned the amount of Cu evaporation so that a mono-atomic layer of Cu would be deposited on the CdTe.

"Efficiencies increased dramatically, from just under one percent to above 12," said Kranz. Their best value was 13.6% for a CdTe cell grown on glass while on metal foils, Tiwari’s team reached efficiencies of up to 11.5%.

Ambitious target: hitting the 20% ceiling

For now, the highest efficiencies of flexible CdTe solar cells on metal foil are still lower than those of flexible solar cells in superstrate configuration on a much more expensive transparent polyimide foil, developed by Tiwaris team in 2011.

"Our results indicate that the substrate configuration technology has a great potential for improving the efficiency even further in the future," said Stephan Buecheler, who co-authored the report.

The team’s short-term goal is to reach 15%, but, Buecheler added, "Im convinced that the material has the potential for efficiencies exceeding 20%."

The next steps will focus on decreasing the thickness of the window layer above the CdTe, including the electrical front contact, which would reduce light absorption and allow more sunlight to be harvested by the CdTe layer. "Cutting the optical losses" is how Tiwari describes it.

The study was supported by the Swiss National Science Foundation and the Competence Center Energy and Mobility of the ETH Domain.

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